This invention relates to a fuel cell system, and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power for vehicle propulsion.
Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution of the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation, assignee of the present invention, and having as inventors Swathirajan et al. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture Of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent damage thereto.
For vehicular applications, it is desirable to use a liquid fuel, preferably a hydrocarbon or alcohol, such as methanol, or gasoline as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: CH3OH+H2Oxe2x86x92CO2+3H2. The reforming reaction is an endothermic reaction, which means it requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442, now U.S. Pat. No. 5,887,276, and 08/980,087, now U.S. Pat. No. 6,077,620, filed in the name of William Pettit in November, 1997, and U.S. Ser. No. 09/187,125, now U.S. Pat. No. 6,238,815, Glenn W. Skala et al., filed Nov. 5, 1998, and each assigned to General Motors Corporation, assignee of the present invention.
For vehicular power plants, the reaction within the fuel cell must be carried out under conditions which preserve the integrity of the cell and its valuable polymeric and precious metal catalyst components. Since the anode, cathode and electrolyte layers of the MEA assembly are each formed of polymers, it is evident that the integrity and/or capabilities of such polymers may be adversely affected if exposed to too high a temperature.
The present invention is directed to an improved method and system to maintain the integrity and capability of the fuel cell stack by detecting a low voltage event and implementing adjustive action. According to the invention, the voltage of individual cells within a fuel cell stack, or the voltage of clusters of cells, is compared to a calibration value. Preferably, multiple calibration voltage values are established based on load. The method and system of the invention are adapted for use in a fuel cell system having a fuel processor which supplies a hydrogen-rich stream to the stack containing fuel cells. In the stack, hydrogen reacts with oxygen to supply electrical power to an external load. By the method of the invention, the voltage of one or more of the cells is monitored. Preferably, the voltage of a cluster of cells is monitored, rather than monitoring individual cells. The monitored voltage is compared to at least one preselected calibration voltage value. A signal is generated if the monitored voltage is less than the preselected value. Preferably, the preselected voltage value is a function of load. More preferably, different preselected values are established for different loads.
In another embodiment, the voltage of one or more cells is monitored; and the monitored voltage is compared to first and second preselected values as a function of load, where the second preselected value is less than the first preselected value. Next, either a first signal is generated if the monitored voltage is less than the first preselected value and greater than or equal to the second preselected value; or a second signal is generated if the monitored voltage is less than the second preselected voltage.
In one embodiment, the first preselected value is selected to correspond to a rate at which the stack is operable to consume the hydrogen-rich stream to satisfy a reduced load. Therefore, when the first signal is generated, the external load is reduced. Preferably when the second signal is generated, indicating a relatively ultra-low voltage condition, the supply of power to the external load is terminated and the fuel cell system is shutdown.
The monitoring and control system of the present invention provides important advantages, particularly in the case where a fuel cell system does not directly monitor the rate of hydrogen flow to the fuel cell. In a fuel cell system, it is important to match the load being demanded of the system with the rate at which reformate gas is supplied to the fuel cell. If it is attempted to draw more current out of the fuel cell than it is capable of supplying because there is not enough hydrogen to create electrical power, this may exceed the acceptable working range of the fuel cell stack and adversely affect the integrity of the stack. Exceeding the acceptable working range of the stack may result in breakdown of the membrane, polymer components. Therefore, it is advantageous to have a control method which provides an early indication where an amount of current is being drawn corresponding to the load demanded, yet stack voltage begins to drop.
Advantageously, the present monitoring and control method is adaptable to, and easily implemented in, existing fuel cell systems. The present method can be implemented in existing fuel cell controllers. In addition, the present monitoring and control method is useable with a variety of fuel cell systems.